1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a torque detecting mechanism for detecting the torque of a turntable of a polishing apparatus for polishing a workpiece such as a semiconductor wafer or the like to a flat, mirror finish.
2. Description of the Prior Art
As semiconductor devices become more highly integrated in recent years, circuit interconnections become thinner and the distances between such interconnections also become smaller. Photolithographic processes for producing interconnections that are 0.5 .mu.m wide or smaller particularly, require a flat image-focusing plane for the stepper because the depth between focal points is small.
Therefore, it is necessary to make the surface of a semiconductor wafer flat before fine circuit interconnections are formed thereon. According to one customary process, semiconductor wafers are polished to a flat finish by a polishing apparatus.
One conventional polishing apparatus comprises a turntable with an abrasive cloth attached to its upper surface, a top ring disposed in confronting relationship to the upper surface of the turntable, and a dressing device for dressing the abrasive cloth, the turntable and the top ring being rotatable at respective independent speeds. The top ring is pressed against the turntable to impart a certain pressure to the turntable, with a workpiece to be polished being interposed between the abrasive cloth and the top ring. While an abrasive fluid is flowing onto the upper surface of the abrasive cloth, the surface of the workpiece is polished to a flat, mirror finish by the abrasive cloth during relative rotation of the top ring and the turntable.
Japanese laid-open patent publication No. 6-216095 discloses a technique for monitoring a torque, i.e., a polishing resistance, acting on a top ring with a torque meter. When the polishing of a semiconductor wafer progresses and the semiconductor wafer is planarized, the polishing resistance is quickly reduced. Therefore, it is possible to detect when the polishing process comes to an end by detecting the polishing resistance with the torque meter. However, the above publication is silent as to details of the torque meter, and is not clear about what type of torque meter is used and how the torque meter is installed to measure the torque applied to the top ring.
U.S. Pat. No. 5,069,002 reveals a technique for detecting an end of the planarization of a semiconductor wafer based on an abrupt reduction in the polishing resistance when the semiconductor wafer is polished to a flat finish. According to this publication, an abrupt reduction in the polishing resistance at the time the semiconductor wafer is planarized, i.e. an end of planarization, is recognized by detecting a change in a drive current supplied to one or both of motors for rotating a top ring and a turntable, respectively.
In order to select a motor that is not overly designed, it is necessary to detect the torque of a motor which rotates the turntable of a polishing apparatus. It has been customary to calculate the torque of such a motor based on a current flowing through the motor because an ordinary torque detector cannot be used with the polishing apparatus due to the large amount of space required for installation thereof. However, since the motor is controlled by an inverter, the current flowing through the motor tends to be affected by the characteristics of the inverter. When operating conditions of the polishing apparatus are changed, the characteristics of the inverter are more likely to affect the current flowing through the motor, making it difficult to recognize the characteristics of the motor. Furthermore, many parameters that are needed for calculation of the torque present difficulties for detecting an accurate value of the torque.
A torque detector for detecting the torque of the turntable or top ring of a polishing apparatus should preferably be small in size and light in weight because it would be used in a clean room. Such a torque detector is required to be electrically connected in a manner not to product dust particles. However, there has not been available a torque detector which does not produce dust particles, and which is small in size and light in weight, can easily be electrically connected to a rotating mechanism of a polishing apparatus, and has a high level of torque detecting accuracy.